A secondary battery has been used as a power source in various fields because it is repeatedly chargeable and rechargeable.
A secondary battery is used in a hand-held device such as a cellular phone, a laptop computer, a digital camera, a video camera, a tablet computer, an electric-powered tool, etc.
The secondary battery is also used in various electric-driven power devices such as an electric bicycle, an electric motorcycle, an electric vehicle, a hybrid vehicle, an electric ship, an electric airplane, etc.
The application field of the secondary battery has been expanded to a power storage device used to store power generated using renewable energy or surplus generated power or an uninterruptible power system (UPS) for stably supplying power to various information communication devices including a server computer, a communication base station, etc.
The secondary battery is connected to a load device through a power conversion device such as an inverter. The load device means an arbitrary device using power stored in the secondary battery.
The load device includes a control system. The control system is provided with an output parameter for the secondary battery from a BMS system.
The output parameter is a factor indicating discharging capability or charging capability of the secondary battery, and is updated according to a state of charge and a temperature of the secondary battery. The output parameter is a maximum current capable of flowing through the secondary battery when the secondary battery is discharged or charged, or an available power calculated from the maximum current.
The control system controls charging and discharging of the secondary battery in a range of the output parameter if the control system is provided with the output parameter from the BMS system. That is, the control system controls the power conversion device, thereby the output of the secondary battery limited to the available power or lower, or the magnitude of charging or discharging current limited to the maximum current or lower.
Meanwhile, a voltage of the secondary battery changes fast as the voltage approaches a discharging lower limit voltage or a charging upper limit voltage.
FIG. 1 is a graph showing a pattern of a voltage change in case of a lithium secondary battery having a capacity of 36 Ah discharged with a constant current of 360 A when a state of charge of the lithium secondary battery is 20%.
As is shown, as a voltage of the secondary battery approaches a discharging lower limit Vmin (see a dot A), a deceleration rate of the voltage abruptly increases. This phenomenon also occurs in a charging situation of the secondary battery. That is, when the secondary battery is charged with a constant current of 360 A, an acceleration rate of the voltage abruptly increases as the voltage approaches a charging upper voltage.
Accordingly, if the voltage change rate of the secondary battery starts increasing, the BMS system needs to derate an output parameter to be provided to the control system. By doing so, it is possible to prevent the secondary battery from deviating from an operating voltage range in the latter half of discharging or charging period.